In recent years, efforts to provide highly integrated and downsized image pickup elements such as a CCD and CMOS have been made. Corresponding to that, an image pickup device such as a digital still camera and a video camera each employing a device such as a CCD and CMOS is required to exhibit higher performance and to be downsized as the entire device.
With the popularization of digital still cameras, digital still cameras have been used at various situations. As the result, efforts to enhance the portability of digital cameras, namely efforts to reduce their size and weight have been made.
In digital still cameras, achieving higher variable power and wider angle of view increases the degree of freedom of shooting. For example, it enables to shoot a subject from a position closer than ever to the subject and enables to shoot a wide area even under the situation that the position of the subject is close to the operator such as indoor shooting, which are advantageous.
As a compact and high-variable-power zoom lens which is suitable for a digital still camera and a video camera, four-group zoom lenses are known (for example, JP-A Nos. 2005-164905, 2008-309993, 2009-025366, and 2010-134108), wherein the zoom lens is composed of leans groups having positive, negative, positive and positive refractive power in this order from the object side and an interval of each neighboring lens groups are changed for varying the power.
However, in the zoom lens disclosed in JP-A No. 2005-164905, the function of varying power is mainly acted by the third lens group and the third lens group is required to be moved greatly in order to realize high variable power, which enlarges the whole optical system in length. Thereby, the diameter of the forefront lens becomes large and it brings enlarged size of a device.
Though JP-A No. 2008-309993 provides a simple lens construction, the angle of view at the wide-angle end is as small as 50° to 60° and the variable power ratio is as small as about ×4.7. Further, since the first lens group is moved greatly for varying power from the wide-angle end to the telephoto end, the length of the whole lens system becomes long and it brings enlarged size of a device.
Though JP-A No. 2009-025366 realizes an optical system with excellent optical performance wherein the variable power ratio is about ×10, the angle of view at the wide-angle end is as small as about 60°. The optical system has a large number of lenses in order to obtain high variable power ratio and excellent optical performance, which increases the cost.
JP-A No. 2010-134108 provides an optical system with a variable power ratio of about ×10 and an angle of view at the wide-angle end of about 80°. However, a stop is moved in the optical system, which increases the number of movable parts in the optical system. Therefore, a mechanism for moving a movable group is required and it brings enlarged size of a device.